Convective heat transfer furnace and method



CONVECTIVE HEAT TRANSFER FURNACE AND METHOD Filed May 28, 1958 10Sheets-Sheet 1 Till- INVENTORS DONALD BEGG S JACK HUEBLER BY BARREL 8.SABIN JOHN c. SCARLETT ZURIZ;

Feb. 13, 1962 D. BEGGS ETAL 3,021,236

CONVECTIVE HEAT TRANSFER FURNACE AND METHOD l0 Sheets-Sheet 2 Filed May28, 1958 II I ' INVENTOR.S DONALD BEGGS JACK HUEBLER BY BARREL B-SABINoa-a c. cm mefl Twin Feb. 13, 1962 D. BEGGS ETAL 3,021,236

CONVECTIVE HEAT TRANSFER FURNACE AND METHOD Filed May 28, 1958 10Sheets-Sheet 3 INVENTORS DONALD BEGGS JACK HUEBLER BY DARREL B. SABINJOHN c. SCARLETT JOHN J. TUR\N ATT ye Feb. 13, 1962 D. BEGGS ETAL3,021,236

CONVECTIVE HEAT TRANSFER FURNACE AND METHOD Filed May 28, 1958 10SheetsSheet 4 i x g LO Ft A A", I (J V E'\ [Q g! J g E E Q I U1 El P-iINVENTORS DONALD B EGGS JACK HUEBLER BY DARREL B- SABIN JOHN c. SCARLETTATT Feb. 13, 1962 D. BEGGS ETAL 3,021,236

CONVECTIVE HEAT TRANSFER FURNACE AND METHOD Filed May 28, 1958 l0Sheets-Sheet 5 MAXIMUM PERMISSABLE VIBRATION AMPLITUDE OF VIBRATION VINVENTORS FLUID VELOCITY DONALD BEGGS JACK HUEBLER BY DARREL B. SABINJOHN C. CARLETT TIES- MJOHN A1T mfg Feb. 13, 1962 D. BEGGS ETAL3,021,236

CONVECTIVE HEAT TRANSFER FURNACE AND METHOD Filed May 28, 1958 10SheeosSheet 6 J36 T 10 R Ill Emil- INVENTORS DONALD B EGGS JACK HUEBLERBY BARREL 6. SABIN JOHN c. SCARLETT CONVECTIVE HEAT TRANSFER FURNACE ANDMETHOD Filed May 28. 1958 10 Sheets-Sheet 8 Twi INVENTORS DONALD B EGGSJACK HUEBLER BY DARREL B. SABIN JOHN c. SCARLETT Feb. 13, 1962 D. BEGGSETAL 3,021,236

CONVECTIVE HEAT TRANSFER FUR Filed May 28, 1958 NACE AND METHOD l0Sheets-Sheet 9 w c: E I! LL! 0. E LLI P TIME C m a: E T 4 F A m f Ll-lD- E I- PERMISSABLE VARlATlON IN FURACING TIME To T 1 i I TIME 1EMISSIVITY 0-3 LU g T EMISSIVITY 0.2 g t E 'ss1vnY 0.1 E 1 AT 5 A l uoNw 'vs ss EMISSIVITY 0.2 \5 JAcv: DHUEBLER C l BARREL 5 SABIN T 04 BYJOHN c. s'cAmETT TIME JON; J%RTN:lN/%

Feb. 13, 1962 D. BEGGS ETAL 3,021,236

CONVECTIVE HEAT TRANSFER FURNACE AND METHOD Filed y 1958 10 Sheets-Sheet10 u) I LU 1.- .040 E- E I Lu .030 N. W Z 5 .020 9 .olo

I000 "00 I200 I300 I400 I500 ANNEALING TEMPERATURE E FILED- INVENTORSDONALD BEGGS JACK HUEBLER BY DARREL B- SAB\N JOHN C. SCARLETT it Sites3,021,236 CONVECTIVE FEAT TRANSFER FURNACE AND METHOD Donald Beggs,Toledo, Jack Huebler, Sylvania, Darrel B. Sabin, Martin, and .l'ohn 6.Scarlett and .iohn J. Turin, Toledo, Ohio, assignors, by mesneassignments, to Midland-Ross Corporation, Cleveland, Ohio, a corporationof Ohio Filed May 28, 1958, Ser. No. 738,912 13 Claims. (Cl. 148-13)This invention relates to a convective heat transfer furnace and method,and, more particularly, to a furnace and method wherein strip work isheated rapidly to a desired temperature, and a heated compressible fluidis circulated in contact therewith, at a rate sufficiently high thatconvective heat transfer predominates over radiative heat transfer. Thisapplication is a continuation-in-part of application Serial No. 530,858,filed August 26, 1955, now abondoned.

A recent development in th art of heat treatment involves rapid heatingof work, usually metal, during a short residence time in a heating zonemaintained at a temperature substantially above the desired worktemperature. Rapid heating of this type achieves numerous advantagesincluding not only economic advantages such as substantial reduction inthe floor space that is required to process any given tonnage per hourand smaller furnacing apparatus requirements, but also improvedmetallurgical results, such as better grain characteristics.

A brief consideration of the factors controlling radiant and convectiveheat transfer provides a theoretical basis for the experimentallyobserved fact that radiant heat transfer predominates in a furnacingoperation of the type described in the preceding paragraph, which ishereinafter referred to as high thermal head furnacing. In high thermalhead furnacing, which usually is conducted as a continuous operation,the temperature difference between work discharged from the heatingchamber and the walls of the chamber is ordinarily in excess of 500 F.,and often as much as 1000 F. It is known in the art that the rate ofradiant heat transfer from the Walls of the chamber to the Work passingthereth-rough is a direct function of the difference between the fourthpower of the absolute temperature of the walls and the fourth power ofthe absolute temperature of the work, while the rate of convective heattransfer from the atmosphere in the furnace to the work is a directfunction of the difference between the first powers of these absolutetemperatures, assuming the atmosphere temperature to equal the furnacewall temperature. It has been shown in a specific typical instance (seeIndustrial and Engineering Chemistry, volume 40 No. 6, pages 1995 andfollowing), that at about 2000 F., approximately 80 percent of the heattransferred to work is by radiative heat transfer, and only about 20percent is by convective heat transfer.

While excellent results have been achieved in many instances using highthermal head furnacing, numerous difficulties have had to be overcome.What has been denominated the geometry of radiative heating isresponsible for substantial temperature variations in the work, unlesssuitable provisions are made to compensate therefor. A briefconsideration of the high thermal head furnacing of plate stock, even ina heating chamber that is generally circular in cross-section, providesa specific example of one reason why the geometry of radiative heatingcauses temperature variations in the work. During such furnacing, thelongitudinal edge surfaces of the work are heated by radiative heattransfer from the walls of the furnace laterally adjacent the edges, andupper and lower surfaces adjacent the edges are heated by radiative heattransfer from the walls of the furnace thereabove Patented Feb. 13, 1962and therebelow. In contrast, the surfaces of the work near the centerthereof are heated only by radiative heat transfer from the walls of thefurnace thereabove and therebelow. It has been found in actual practicethat, as a result of the foregoing fact, temperature differences betweenthe center of a plate and either edge thereof may be as much as 200 F.

It is known that the rate of radiative heat transfer is a directfunction of the emissivity of the surface of the material being heatedand it has been found that the emissivity of a surface depends not onlyupon the material, but also upon the surface condition thereof. Forexample, a carbonaceous surface deposit may double the emissivity of apolished brass surface, and roughening of the surface may have alikeeffect.

The rate at which work is heated during high thermal head furnacing asordinarily extremely high. While rapid heating has advantages asdiscussed above, serious control problems arise if it is attempted toheat treat strip work rapidly in apparatus where radiative heat transferpredominates. A major reason for the extreme difficulty of control withstrip work is that no practical way is known to measure striptemperature during such heating. Since temperature differences frompoint to point in the strip, resulting from the geometry of radiativeheating, from variations in emissivity and gauge of the strip, fromvariations in residence time, or from other such factors cannot bemeasured, there is no practical way to eliminate the temperaturedifferences during such furnacing. Apparatus according to the invention,however, in one embodiment, makes it possible to heat strip workrapidly, and substantially to eliminate temperature differences frompoint to point on the strip.

In the heat treatment of cold worked brass strip, it is known to beextremely diflicult to achieve uniform grain size by high thermal headfurnacing. Brass grain size after annealing, for example, is known to bea function of annealing temperature, other factors being equal. It isbelieved that variations in emissivity, in thickness, or in both,whether across a width or from point to point along the length of thestrip, are responsible for sufliciently wide temperature variationsduring such furnacing to cause the non-uniform grain size. In addition,the inability to compensate adequately for temperature differencescaused by the geometry of radiative heating, or even slight variationsin residence times, is believed to contribute to the non-uniform grainsize.

The instant invention is based upon the discovery of a convective heattransfer furnace and method enabling the rapid heating, predominantly byconvective heat transfer, of continuous strip work. Cold worked brassstrip, for example, can be heat treated continuously according to theinvention to substantially uniform grain size, indicating substantiallyuniform temperatures and times at temperatures for all parts of thestrip.

It is, therefore, an object of the invention to provide improvedapparatus for the rapid heating of continuous strip work.

It is a further object of the invention to provide an improved methodfor the heat treatment of continuous strip work.

Other objects and advantages will be apparent from the description whichfollows, reference being had to the accompanying drawings, in which-FIG. 1 is a diagrammatic representation of a continuous strip processingline including heat treating apparatus according to the invention;

FIG. 2 is a view in vertical section of apparatus according to theinvention for rapid heating of continuous strip;

FIG. 3 is a vertical sectional view showing a pre-heat section of theapparatus of FIG. 2, and including an inlet section which minimizesvibrational eflects on continuous strip work caused by the introductionof a compressible fluid flowing at high velocity adjacent thereto duringheat transfer between the work and the fluid;

FIG. 4 is a vertical sectional view showing the heating portion of theapparatus of FIG. 2;

FIG. 5 is a vertical sectional view showing the cooling portion of theapparatus of FIG. 2;

FIG. 6 is a view in vertical section similar to FIGS. 3-5 showing amodified heating or cooling apparatus according to the invention;

FIG. 7 is a vertical sectional view similar to FIG. 6 showing a furthermodified form of cooling apparatus;

FIG. 8 is a vertical sectional view of a compressible fluid inletportion of a heating or cooling chamber according to the invention, andshowing a modified form of inlet for minimizing vibrational effects oncontinuous strip work caused by the introduction of a compressible fluidflowing at high velocity adjacent thereto during heat transfer betweenthe work and the fluid;

FIG. 9 is a diagram showing the advantage of inlets according to theinvention for minimizing vibrational effects on continuous strip work;

FIG. 10 is a vertical sectional view showing a modified inlet accordingto the invention;

FIG. 11 is a view in vertical section showing a still further modifiedinlet;

FIG. 12 is a vertical sectional view of still another inlet;

FIG. 13 is a view in vertical section of a further modified inlet;

FIG. 14 is a vertical sectional view of an additional .inlet;

FIG. 15 is a view in vertical section on an enlarged scale of thecompressible fluid discharge portion of the cooling apparatus shown inFIG. 5, and showing details of mechanism for preventing undesired flowof compressible fluid from the cooling apparatus to heating apparatus;

FIG. 16 is a vertical sectional view of a modified form of coolingapparatus according to the invention showing details of means for rapidcooling of strip work;

FIG. 17 is a time-temperature diagram showing heating curves for stripwork heated by a compressible fluid flowing at two different highvelocities in apparatus ac cording to the invention, and by radiativeheat transfer in accordance with the prior art;

FIG. 18 is a diagram similar to FIG. 17, but on an enlarged scale,showing the portions of the three heating curves near the desired finalstrip temperature;

FIG. 19 is a diagram similar to FIG. 18, but with only two of the threecurves represented and showing in addition, the effect of variations inemissivity of the strip work; and

FIG. 20 is a diagram showing average grain size to be expected in 70-30brass strip work as a function of annealing temperature if all otherfactors affecting grain size remain constant.

Referring now in more detail to the drawings, and particularly to FIG.1, a specific continuous strip processing line includes a pair ofpay-off reels 20, serviced by elevators and loaders 21, a pullover roll22, a stitching unit 23, a cleaner 24, a looping tower indicatedgenerally at 25, a tensiometer 26, a heat treating unit indicatedgenerally at 27, a pickling unit 28, an exit looping tower indicatedgenerally at 29, a shear indicated generally at 30, and a reel re-winder31. Continuous strip work 32 is shown in all parts of the processingline.

Referring now to FIG. 2, the heat treating unit 27 in the stripprocessing line of FIG. 1 comprises a rapid convective preheatingapparatus indicated generally at 33, a rapid convective heatingapparatus indicated generally at 34, and a rapid convective coolingapparatus indicated generally at 35. Strip 32 entering the unit 27passes around a roll 36, vertically upwardly through the preheatingapparatus 33, around a roll 37, around a roll 38,

and vertically downwardly through the rapid convective heating apparatus34 and the rapid convective cooling apparatus 35. Strip discharged fromthe cooling apparatus 35 passes around a roll 40 through a liquidatmosphere seal indicated generally at 39, and then is led out of theunit to the looping tower 28.

The liquid atmosphere seal 39 comprises a chamber 41 which is both aplenum chamber, as subsequently described in more detail, and a liquidseal container. A liquid 42, which is preferably water, fills the lowerportion of the chamber 41, surrounding the roll 40, and is forcedupwardly by compressible fluid pressure in the chamber 41 into a stripoutlet 43 and an overflow 44.

In the heat treatment of brass strip, temperatures below about 800 F.are sub-critical in that the surfaces of the strip are not damaged bypassing over rolls while at such temperatures, and grain size issubstantially unaffected. At temperatures higher than about 800 F.,however, zinc is vaporized from brass strip, and tends to deposit uponavailable surfaces, for example, the surface of a roll. Therefore, ifbrass strip passes over or around a roll while at a temperature higherthan about 800 F., the zinc vaporized tends to build up in volcaniclikedeposits on the surface of the roll. Such deposits, after a short periodof operation, cause permanent dam age to strip passing thereover.

The foregoing result may be avoided by passing the strip work throughthe furnace in a straight line, eliminating the passage over or aroundrolls, but practical limitations on furnace size limit the maximumproduction rate of a straight line furnace. In a furnace of any givenphysical size the production rate may be increased by making two passesthrough the length, but this requires reversal of direction of the striparound rolls and, as pointed out above, zinc deposits on the rollsurfaces if they are located at a point where the temperature is higherthan about 800 F.

In the apparatus 27 this dilemma is overcome by the utilization of thepre-heat chamber 33 extending upwardly, in which the strip 32 ispre-heated to a temperature not higher than 800 F. so that its directionis changed by the rolls 37 and 38 without the danger of zinc deposit,and the strip 32 then moves downwardly through the heating apparatus 34and cooling apparatus 35, where it is heated to the desired temperatureand cooled without contacting a roll. The use of the pro-heating chamber33 thus provides for a maximum length of heat applying distance and timeand, thus, maximum production for a furnace of any given height.

However, the pre-heating apparatus 33 can be eliminated, if desired, andthe heating apparatus 34 and cooling apparatus 35 operated in preciselythe same way as will be described below to achieve identical results,except, of course, at a lower rate of strip travel and production.

The specific rapid convective pre-heating apparatus 33 (FIG. 3)comprises a heating chamber 45, which, in the specific embodiment of theinvention shown, is of the duct or conduit type, and external duct work46 including a fluid heater 47 and a blower 48 for circulating acompressible fluid through the heating chamber 45. The compressiblefluid moves upwardly through the duct 46 in the direction of the arrow,and is discharged therefrom into a plenum chamber 49, and thence flowsthrough an inlet indicated generally at 50 and downwardly through theheating chamber 45 contrary to the direction of movement of the strip 32therethrough. Compressible fluid discharged from the chamber 45 enters areturn plenum chamber 51 and passes from there through a return duct 52to the low pressure side of the blower 48.

The inlet 50, in the specific embodiment of the invention shown in FIG.3, is a trough-shaped member 53 which converges in the direction offluid flow toward the heating chamber 45, and is supported symmetricallywith respect to the work, to the heating chamber 45. and

to a flange 54- of the heating chamber 4-5. Support for thetrough-shaped member 53 is provided by laterally spaced, vertical braces55 Welded or otherwise rigidly attached both to the trough-shaped member53 and to the passage provided by its open end and through the minorpassages formed by slots between the braces 55. The braces 55 mayconsist of slotted plates, spaced arms or similar structures providingfor structural mounting and the ingress of fluid in lesser quantity intothe chamber 45.

It has been found, however, that the benefit of the particular inlet 50is retained even though the member 53 is supported in a cockedrelationship with respect to either or both the work 32 and the flange54, or off center.

Referring now to FIG. 4, the specific rapid convective heating apparatus34 is substantially identical with the pre-heat chamber 33, except thatit is positioned in inverted relationship so as to providecountercurrent flow of compressible fluid relative to the strip work 32.The apparatus 34 comprises a heating chamber 56, ducts 57 forcirculating a compressible fluid heated in a heater 58 through thechamber 56, and a blower 59, for circulating the compressible fluid. Thecompressible fluid moves downwardly through the duct 57 in the directionof the arrow, and is discharged therefrom into a plenum chamber 60, andthence flows through an inlet indicated generally at 61 upwardly throughthe chamber 56. Fluid enters the inlet 61 both through its openconverging end passage 61a and its minor passages or slots 61b.Compressible fluid discharged from the chamber 56 enters a return plenumchamber 62 and passes from there through a return duct 63 to the lowpressure side of the blower 59. The inlet 61 is identical in itsstructural details and operation with the inlet 56 of FIG. 3.

Referring now to FIG. 5, the specific rapid convective cooling apparatus35 is similar in construction to the pre-heat apparatus 33 and also tothe rapid convective heating apparatus 34, comprising a cooling chamber64, ducts 65 for circulating a compressible fluid cooled in a heatexchanger 66 through the chamber 64- and a blower 67 for circulating thecompressible fluid. The compressible fluid is blown through the duct 65in the direction of the arrow, and is discharged therefrom into theplenum chamber portion of the chamber 41, and thence flows through aninlet indicated generally at 68 upwardly through the chamber as. Thefluid enters the inlet 68 through its open converging end 68a andthrough its econdary openings 68b. Compressible fluid discharged fromthe chamber 64 enters a return plenum chamber 69 and passes from therethrough a return conduit 7G to the heat exchanger 65 and then to the lowpressure side of the blower 67. The inlet 68 is identical in itsstructural details and operation with the inlet 5 of FIG. 3.

In the specific heat treating unit 27 it has been found to beadvantageous to employ direct heating in the heaters 47 and 58, anddirect cooling in the heat exchanger 66 to maintain the desiredcompressible fluid temperatures. Direct heating can be accomplished inthe heaters 47 and 58 by combustion therein, in contact with therecirculated fluid, of gas or oil with a desired quantity of air, andproduces flue gas as the fluid. Direct cooling in the heat exchanger 66can be accomplished by spraying water into the heat exchanger, orflowing water therethrough, for example over Berl saddles, in contactwith the compressible fluid therein. It has also been found to beadvantageous to operate the unit 27 so that there is a small flow ofcompressible fluid from the plenum chamber 6% of the apparatus 34 intothe plenum chamber 69 of the apparatus 35, as well as from the plenumchamber 62 of the apparatus 34- into a transfer chamber 71 (see FIG. 2),and thence into the plenum chamber 4-9 of the pre-heat chamber 33. Whenthe unit 27 is operated in such way there is no opportunity for thecompressible fluid in the apparatus 34 to be cooled either by thecompressible fluid in the cooling apparatus 35 or in the pre-heatapparatus 33.

It is also possible to introduce any desired compressible fluid, otherthan flue gas, or another atmosphere, which may be oxidizing, neutral orreducing, into the pre-heat apparatus 33, the heating apparatus 34 andthe cooling apparatus 35 by supplying the desired compressible fluid,and any necessary make-up, to each circulating system. In such case,when the atmosphere is not flue gas, indirect heating should be employedin the heaters 47 and 58, for example by means of radiant tubes, orelectric heating elements, in order to avoid contamination of thecompressible fluid. Either direct or indirect cooling can be utilized inthe heat exchanger 66 provided that suitable precautions are taken inthe former instance to avoid contamination of the compressible fluid bythe coolant. For example, when water is used as a direct coolant, thewater can be recirculated and indirectly cooled to avoid theintroduction thereinto of oxygen or other material that might be presentas absorbed gas in the liquid water and whose presence as a contaminantmight be undesirable in the system. It may then be desirable todehydrate the compressible fluid if water is an undesirable constituenttherein.

In each of the rapid convective heat transfer devices shown in detail inFIGS. 3-5, compressible fluid is circulated in a closed system. Thisrepresents a preferred arrangement of apparatus according to theinvention. However, as shown in FIG. 6, an open system can also beemployed to provide the desired compressible fluid flow through a rapidconvective heat transfer chamber 72. In the specific apparatus shown inFIG. 6, a blower 73 draws a compressible fluid into a conduit 74 anddischarges the compressible fluid through a duct 75 into a heatexchanger 76, and thence through a duct 77 into a plenum chamber 78. Thecompressible fluid flows from the plenum chamber 78, in the mannerpreviously described, into the chamber 72 through an inlet indicatedgenerally at 7?, which is structurally identical with the inlets 5t), 61and 63 (FIGS. 3, 4 and 5), having a major open converging end 79a andsecondary or minor openings 7%. The compressible fluid supplied to theduct 74 can be air, or any other desired fluid provided from a source(not illustrated). The heat transfer chamber 72 can be a pre-heater,heating apparatus, or cooling apparatus, depending upon whether thecompressible fluid is heated or cooled in the heat exchanger 76, asdescribed.

Referring now to FIG. 7, still another type of open compressible fluidcirculation system is shown. The apparatus of FIG. 7 comprises a coolingchamber 80 having a compressible fluid inlet indicated generally at 81,and identical in its structural details with the inlets previouslydiscussed, having a major, open, converging end 81a and secondaryopenings 81b. Compressible fluid is drawn from a plenum chamber 82,through a duct 83 by a blower 84. and discharged from the blower 84through a duct 85 to atmosphere. Withdrawal of compressible fluid fromthe plenum chamber 82 creates a partial vacuum therein which induces aflow of air from the atmosphere through the inlet 81 and the coolingchamber 80. Because there is no possibility for controlling either thecomposition or the temperature of the compressible fluid drawn into thechamber 80, it is possible only to use this arrangement for rapidconvective cooling, and only when ambient air temperatures aresufliciently low for this purpose. The inlet 81 makes possible extremelyhigh compressible fluid velocities, without disruption of stripstability, and thereby enables extremely rapid convective cooling of thestrip.

A slightly modified inlet similar to those previously discussed isindicated generally at 86 in FIG. 8. The inlet 86 comprises atrough-shaped member 87, converging in the direction of fluid flowtoward a heating or cooling chamber 88 through which the work 32 passes.The

member 87 is supported relative to the chamber 83, in generallysymmetrical relationship to the work, and to the chamber, and held byspaced arms 89. The arms 89 are supported relative to a flange 90 of thechamber 88 by nuts 91 and wing nuts 92. As in the cases of the earlierdescribed inlets, fluid enters the chamber 83 through the major openingthrough the member 87 and through secondary openings between the spacedarms 89. In the specific embodiment shown in FIG. 8 the inlet 86 ispositioned in a plenum chamber 93. The benefit of the particular inlet86, like that of the inlets previously discussed, is retained eventhough the member 87 is supported in a cocked relationship with respectto either or both the work 32 and the chamber 88, or off center.

The various inlet devices discussed above in conjunction with FIGS. 3-8are particularly advantageous in connection with the so-called stripinstability and flutter problem. Specifically, in the rapid convectii eheat transfer furnacing of strip work, it has been found to be diflicultto achieve, in the heating and cooling chambers, the compressible fluidvelocities necessary to raise the convective heat transfer coeflicientsufliciently that the ratio of convective heat transfer to radiativeheat transfer is at least 2:1. Instead, it has been found that, as thecompressible fluid velocity is increased, strip work in a heatingchamber such as a duct, becomes unstable mechanically and begins tovibrate. The vibration at first is not a true resonant vibration becauseit seems to have no regular frequency, but as wind velocity is increasedstill further, the amplitude of random vibration increases, until thecondition is reached when the vibration amplitude is sufficient that thework strikes the walls of the heating chamber or duct thus irreparablymarring the surface of the work.

In some instances, as the compressible fluid velocity is increased, thestrip is forced into contact with one ofthe walls of the heatingchamber, or into a twisted condition sometimes in contact with bothopposite walls. These positions are limiting conditions, obviouslyuseless for the present purpose since it is impossible to transfer heatby fluid convection to the surface of a strip in contact with a wall, orto transfer heat uniformly to the surface of the strip when the velocitycontour of the convection fluid within the duct has been renderednon-uniform by the erratic position of the strip work within the duct.

It is furthermore recognized that even if the limitations of movementintroduced by the proximity of the walls were removed, and thecompressible fluid velocity through the heating chamber increased, forany specific tension and strip thickness, there would occur a trueresonant vibration. This resonant vibration, called flutter, is usuallycharacterized by the vibration amplitude rising almost without limit asthe fluid velocity is slightly increased beyond a. certain criticalmaximum. Flutter causes such large destructive forces to come into playas to cause violent rupture of the strip work. When it is possible tostabilize the strip sufficiently in a duct so that the convection fluidvelocity may be increased to the velocity which causes flutter, theultimate limitation in convective heat transfer to the strip is attainedat a velocity slightly smaller than that inducing flutter. The unstablevibrations discussed above, however, in ordinary installations, preventthe utilization of compressible fluid velocities even approaching theconvection fluid velocity capable of causing resonant flutter.

The vibration condition in previously known strip heating chambers isrepresented by curve B of FIG. 9, which is a plot of amplitude ofvibration of strip work passing through a duct-type heating chamberagainst fluid velocity in such chamber. It will be noted that curve Bpasses through the origin of the plot, but has a relatively constantpositive slope, indicating that amplitude of vibration is a direct,almost linear function of fluid velocity through the duct, at least upto dotted line A, which represents a maximum permissible amplitude ofvibration for any particular duct, being a vibration amplitudearbitrarily selected for practical reasons equal to one-half the ductthickness. Curve B is typical for heating chambers having straight orflared fluid inlets, including Venturi trough and bell flares, but withno means for equalizing pressure on opposite sides of the strip, assubsequently discussed in detail.

Amplitude of unstable vibration of work in strip form as a function offluid velocity in a duct-type heating chamber provided with one of theinlets shown in FIGS. 38, and described above in connection therewith,is represented by curve C of FIG. 9. It will be observed that curve C,throughout the range of permissible fluid velocities, is displacedsubstantially to the right of curve B, indicating that at anypermissible maximum amplitude of unstable vibration, a substantiallyincreased convection fluid velocity is permissible without an increasein vibration amplitude. Therefore, using any of the inlets shown inFIGS. 3-8, higher fluid velocities through the duct heating chamber arepermissible. Since the effective convective heat transfer coeflicient isa direct function of fluid velocity, and since the amount of heattransferred by convective heat transfer is a direct function ofeffective convective heat transfer coeflicient, a higher ratio ofconvective heat transfer to radiative heat transfer, other factors beingequal, is possible in furnacing apparatus provided with the inlets shownin FIGS. 3-8, than in such apparatus provided with conventional inlets.

Curves B and C of FIG. 9 represent amplitude of unstable vibration as afunction of fluid velocity through a heating or cooling chamber forconventional inlets and for inlets as shown in FIGS. 3-8, respectively,other variables being held constant. In FIG. 9 the dotted ver tical lineV, represents the convective fluid velocity in contact with the striprequired to cause the resonant flutter variation. It has been found,however, that the fluid velocity, V, required to cause resonant fluttervibration in strip work is also a complex function of the tensionapplied to the strip passing through a duct heating chamber, beingincreased as the tension is increased.

Once the problem of unstable pre-flutter vibration has been eliminatedby employing one of the inlets of the present invention, the fluidvelocity required to induce flutter in a specific instance may beincreased by raising the tension force applied to the strip work 32,passing through the heat treating unit 2?.

In another aspect, therefore, the invention contemplates the provisionof means, such as the tensiometer 26 shown in FIG. 1, for maintainingthe work 32 under high tension as it passes through a heating or coolingchamber. The maximum permissible tension that can be applied to heatedwork under any given set of conditions depends upon many factors. Forexample, in the case of brass strip, it has been found that the work islikely to be unsatisfactory, for example because of permanent mechanicaldistortion or gauge inaccuracy, if the work is elongated more than aboutA of 1 percent during the course of a furnacing operation such as anannealing. In any event, however, in one embodiment, the inventioncontemplates the provision of means, such as the tensiometer 26, formaintaining the strip work 32, as it passes through a heating or coolingchamber, under high tension ranging from about 50 percent to the limitof percent of the maximum tension permissible under the conditionsprevailing.

Although it is not fully understood why the inlets shown in FIGS. 38reduce the amplitude of vibration and provide mechanical strip stabilityat any given fluid velocity below that inducing flutter as shown in FIG.9, it is desired to present a theoretical explanation for thisphenomenon in order to make as full and complete as possible adisclosure of this aspect of the invention. The following theoreticalexplanation, therefore, is presented solely for the purpose of furtherillustrating and disclosing, and is in no way to be construed as alimitation upon the invention. It will be observed in FIGS. 3 and 4, forexample, that the strip work 32 passing through the heating chamber 45or 56 may be considered to be a diaphragm separating such heatingchamber into two portions. Whenever a slight pressure differentialoccurs between the two sides of the diaphragm (strip work), there is atendency for the diaphragm to move in the heating chamber to balance thepressures. Such movement, however decreases the cross-sectional area ofthe low pressure side of the diaphragm, thus tending instantaneously toincrease the velocity of compressible fluid there, and still furtherdecrease the pressure. The further pressure decrease causes stillfurther movement of the diaphragm, with the result that any pressureinequality starts a chain of dynamically unstable consequences whichexaggerates the effect of the original difliculty.

It is believed that the effectiveness of the inlets shown in FIGS. 38 instabilizing the strip and reducing the amplitude of vibration at anygiven fluid velocity smaller than that causing flutter demonstrates thatpressure inequalities between the two sides of the diaphragm are mostlikely to be caused by the dynamic conditions prevailing at the fluidinlet to the heating or cooling chamber.

Each of these inlets provides a plurality of converging passageways forcompressible fluid from a plenum chamber or from atmosphere to a heatingor cooling chamber. Each inlet also provides at least one major passageand minor alternate passages from the plenum chamber or from atmosphereto the interior of the heating or cooling chamber near the fluid inletend thereof. The minor passages are, in essence, pressure stabilizers,in the sense that compressible fluid from the plenum chamber can flowtherethrough to either side of the diaphragm (strip work) with theresult that the effects of pressure inequality between the two sides ofthe diaphragm are counteracted by flow of fluid from the plenum chamberor from atmosphere to the low pressure side of the diaphragm, withresulting strip stabilization and elimination of sub or pre-fluttervibration, because no strip movement is required to equalize pressureson both sides.

Careful study of FIGS. 3-8 reveals that the various inlets which havebeen found to be advantageous have in common the feature that theyprovide a plurality of paths for compressible fluid flow from the plenumchamber exterior of the heating or cooling chamber into the interior ofthe heating or cooling chamber. In the inlets shown in FIGS. 3-8compressible fluid has one flow path interior of a trough-shaped member,which path, itself, converges in the direction of fluid flow, aspreviously described, and also has at least one other path whichconverges, relative to the first path, such other converging path orpaths being defined by the space between the downstream extremity of thetrough-shaped member and the upstream extremity of a flange on theheating or cooling chamber.

Optimum results have been achieved, experimentally, using an inletproviding a plurality of converging paths as shown in FIGS. 38 forcompressible fluid flow from the exterior to the interior of a heatingor cooling chamber, when there has been actual flow of compressiblefluid along a plurality of converging paths. With inlets of the typesshown in FIGS. 38 it has further been found that optimum results, in theform of strip stability, as previously discussed in detail, have beenachieved when the ratio of volume of fluid flowing through theconverging major inlet passages to volume of fluid flowing through theslots or minor passages has been from about 1 /2:1 to about 3:1, mostdesirably from about 2:1 to about 2 /2 :1.

It will be apparent from the foregoing discussion that numerous inletsother than those shown in FIGS. 3-8 can be utilized to increase stripstability by equalizing pressures on either side of strip work in aheating or cooling chamber through which a compressible fluid travels athigh velocity in a direction generally parallel to the strip surfaces.One such inlet is indicated generally at in FIG. 10. The inlet 95 isshown in a plenum chamber 96, and comprises an extension 97 of a heatingchamber 98. Openings 99 in the extension 97 provide minor passages forthe flow of compressible fluid which converge with respect to a majorfluid flow passage into the open end of the extension 97, as indicatedby the arrow. The openings 99 may be continuous or interruptedperipherally, not necessarily being symmetrically arranged with respectto the strip work. The compressible fluid is supplied to the plenumchamber 96 through a supply duct 100. The inlet 95 can be used inconjunction with a plenum chamber in accordance with the showings ofFIGS. 3-6, or with atmosphere as the plenum chamber in accordance withthe showing of FIG. 7, and whether fluid flow is induced by supplyingcompres sible fluid under pressure to the plenum chamber 96 or bydrawing a vacuum on a plenum chamber surrounding the fluid discharge end(not illustrated) of the chamber 98.

Still another embodiment of an inlet according to the invention isindicated generally at 101 in FIG. 11. The inlet 101 comprises atrough-shaped member 102 similar to the member 53 shown in FIG. 3, andwhich converges in the direction of fluid flow toward a heating chamber103. The member 102 is supported symmetrically with respect to the Work32, to the heating chamber 103, and to a flange 104 of the heatingchamber 103 by members 105 welded or otherwise rigidly attached both tothe trough-shaped member 102 and to the flange 104. Fluid flow in thedirection of the vertical arrow is effected through the trough-shapedmember 102 in any of the ways previously discussed. Additional fluidflow from the lateral openings and following paths converging into theheating chamber is induced, for example, by supplying compressible fluidunder pressure to conduits 106, preferably from a common header orplenum chamber (not illustrated). The conduits 106 communicate with aspace between the lower portion of the trough-shaped member 102 and theflange 104, so that the fluid supplied therethrough flows into thechamber 103 in generally the same manner as in the inlets of FIGS. 3-8,except that the rate of such flow can be regulated by varying the fluidpressure in the conduits 106 by any suitable means (not illustrated).The trough-shaped member 102 of the inlet 101 can, if desired, be opento atmosphere, in which case it is useful only as a cooling chamber, orit can be positioned in a plenum chamber and used either for heating orcooling of strip work.

An additional inlet indicated generally at 107 in FIG. 12 is similar tothe inlet 101, comprising a trough-shaped member 108 positioned inspaced relationship with a flange 109 of a heat transfer chamber 110 inthe manner previously described. Minor fluid flow conduits 111 whichprovide paths that are convergent with respect to major fluid flow paths112 through the trough-shaped member 108 on either side of the stripWork 32 are enclosed within a plenum chamber 113 to which a compressiblefluid is supplied under pressure from a duct 114. The upper portion ofthe trough-shaped member 108 is positioned in a plenum chamber 115 towhich a compressible fluid is supplied through a duct 116. By suitablecontrol of the relative pressures of the compressible fluids supplied tothe plenum chamber 113 through the duct 114, and to the plenum chamber115 through the duct 116, the ratio of volume of compressible fluidflowing through the passages 112 to that flowing through the passages111 can be regulated as desired.

Still another modified inlet is indicated generally at 117 in FIG. 13,where it is shown in a plenum chamber 118 to which a compressible fluidunder pressure is supplied through a conduit 119. The inlet 117comprises an extension 120 of a heating or cooling chamber 121. Aplurality of conical fin members 122 extend angularly upwardlytherefrom, on opposite sides of slots 11 123 in the wall of theextension 120. The slots 123 provide a plurality of minor fluid flowpaths which converge with respect to major paths 124 through which fluidflows in the direction of the arrows.

An inlet indicated generally at 125 in FIG. 14 is generally similar tothe inlet 117 of FIG. 13, except that only two conical fins 126 areprovided near the end of an extension 127 of a heat exchange chamber128, so that one minor fluid flow path through slots or openings 129converging with respect to major fluid flow paths 130 is provided oneither side of the strip work 32. In the specific embodiment of theinvention shown, the inlet 125 is positioned in a plenum chamber 131 towhich a compressible fluid under pressure is supplied through a duct132.

The various inlets shown in FIGS. -14 represent modifications of apreferred species of inlet according to the invention wherein there area plurality of converging fluid flow paths. Each of these inlets issuitable for use in a closed system as shown in FIGS. 3-5, or in an opensystem as shown in FIGS. 6 and 7, in place of the inlet specificallyshown in these various figures.

It will be apparent, however, that compressible fluid inlets other thanthose specifically shown and discussed also can be used if they providemeans for equalizing the pressure between the two sides of the stripwork without causing appreciable lateral strip movement in the rapidconvective heating or cooling chamber. In order to be effective pressureequalization must, however, be rapid so that a static pressurecommunication between the two sides of strip in a heating chamber iseffective for strip stabilization if pressure differences are rapidlyeliminated, and any resonant condition in the static interconnectionavoided. When such pressure equalization is accomplished by flowmodulation of a compressible fluid through a plurality of convergingpaths into the interior of a heating or cooling chamber, the necessaryrapidity of pressure equalization is achieved, and any problem ofresonance is eliminated. It is principally for this rea son that thepreferred inlets provide a plurality of converging compressible fluidflow paths.

As has been discussed above, it is usually preferred, in the rapidconvective heating apparatus 34, and also in the rapid convectivecooling apparatus 35, that the compressible fluid flow countereurrent tothe strip 32. When the unit 27 is operated in this way, relatively coldcompressible fluid moving at high velocity adjacent the surfaces of thestrip 32 in the cooling chamber 64* has a substantial tendency tocontinue such movement and to pass from the cooling chamber 64, throughthe upper portion of the plenum chamber 69 and into the plenum chamber60 of the apparatus 34 where it cools the compressible fluid supplied tothe heating chamber 56 of the apparatus 34, thus substantially reducingthe effectiveness thereof.

In a preferred embodiment, apparatus according to the invention includesbufler jets indicated generally at 135 in FIG. for preventing admixtureof relatively cold compressible fluid in plenum chamber 60 from theplenum chamber 69 with hot compressible fluid used for heating (see PEG.2). The buffer jets 135 comprise conduit portions 136 extending acrossthe upper portion of the plenum chamber 69, and generally parallel tothe strip work 32. Spout members 137 are structurally integral with thecon duit portions 136, and are directed downwardly at an angle ofapproximately 40 with respect to the strip work 32, in the embodiment ofthe invention shown. Compressible fluid is withdrawn from the plenumchamber 69 through a pipe 138 and discharged by a pump 139 through apipe 140 to each of the conduit portions 136. The compressible fluid isdischarged through the nozzles 137 against, and across the width of, thestrip work 32 in streams which oppose the flow of compressible fluidfrom the cooling chamber 64 into the plenum 60 (FIG. 2). These opposingfluid streams from the spouts 137 force cold compressible fluid awayfrom the surfaces of the strip 32, and cause a localized fluidcirculation around shroud plates 141, as indicated by the arrows, inaddition to a general compressible fluid movement into the duct '70. Thebuffer jets 135, therefore, prevent cooling of the compressible fluidsupplied to the plenum chamber 60 for heating of strip Work in theapparatus 34, and enable the operation of the apparatus with a generalcompressible fluid flow from the plenum chamber 60 into the plenumchamber 69.

A preferred furnace arrangement for annealing of brass strip is shown inFIG. 2 wherein the heating and cooling are accomplished in a singlevertical pass of the strip to avoid marking the strip due to roll pickupproblems. As shown more clearly in FIG. 16, the strip work 32 enters awater seal 42 prior to passing over a submerged roll 40 therein andthence through duct 43. It is well known that hot strip has a tendencyto warp and buckle when quenched in water; to avoid such distortion, itis necessary to cool the strip to about 250 F. to 300 F. prior toquenching in water. The cooling to 250 to 300 F. cannot be at/toodrastic a rate, or distortion will result; yet, if the cooling rate istoo conservative, or slow, the furnace cooling chamber becomesexcessively long (or has too low a capacity in weight of strip cooledper hour).

It has been found by experiment that even the relatively high convectivecooling rates made possible by the increased fluid velocities resultingfrom this invention are conservative from a strip distortion point ofview. To achieve a maximum cooling rate to the desired temperature,about 250 F. to 300 F., without encountering strip distortion, it ispreferred to use a two-phase convective cooling system where a liquidsuch as water is supplied as a fine mist to the cooling, compressiblefluid used in the cooling chamber.

When water alone is sprayed onto hot strip, heat transfer coeflicientsof the order of 1000 to 2000 B.t.u. per sq. ft. of strip surface perhour per degree F. temperature difference between the strip and watertemperatures are attained. When convective heat transfer cooling isemployed, coeflicients of 20 to 30 are about the maximum attainableshort of encountering resonant flutter in the strip.

Experiments show that brass strip work will tolerate a coolingcoetficient of 50 to 200, when cooled from red heat to about 250 F.,without buckling or distortion.

The two-phase, or mist, convection cooling system will effect coolingcoeflicients of 50 to 200, producing a highly eflicient and compactconvection cooling section. Such coeflicients can be obtained when watermist is injected into a flue gas stream and the mixture is passedlongitudinally through the cooling chamber at velocities below thatwhich causes resonant flutter. By maintaining the liquid content of thecooling compressible fluid relatively low, preferably about 0.1 to 0.5pound of mist per pound of compressible fluid, and circulating asuflicient weight of mixture per weight of strip cooled to maintain aportion of the mixture in liquid form for a substantial portion of thecooling chamber (preferably the compressible cooling fluid leaving thecooling chamber should contain a substantial portion of mist) very rapidand uniform cooling is attained without distortion, and at heat transfercoefficients intermediate those heretofore obtainable in production ofdistortion free strip work.

While the mechanism of mist convection cooling may be somewhatcontroversial, it is believed that the high velocity compressible fluidprevents collection of insulating steam pockets on the strip surface,and that the latent heat of vaporization of the droplets of mistadjacent the strip acts to maintain local fluid temperatures somewhatlower than otherwise attained. It is noted, however, that the degree ofadditional cooling of the circulating compressible fluid by vaporizedmist is not suflicient by itself to explain the higher cooling ratesattained.

In FIG. 16, the compressible fluid within the duct 70 is admitted to aheat exchanger 142 so that the fluid flows upwardly through theheatexchanger to a fluid discharge near the top of the exchanger and passesthrough a duct 143 to the low pressure side of the blower 67. Water orother vaporizable liquid that is sufliciently free of absorbedcontaminants, as described, is supplied to a pipe 1 from any suitablesource (not illustrated), and passes from thence tlrough a pipe 145 intothe upper portion of the heat exchanger 14?. through which the liquidflows in direct contact with, and countercurrent to, the compressiblefluid. An overflow pipe 146 is provided near the bottom of the heatexchanger 142, and below the cornpressible fluid inlet to the heatexchanger. A desired liquid level is thus maintained in the bottom ofthe exchanger 142 to provide a gas seal, and excess liquid is dischargedthrough the overflow pipe 1% and either discarded, or cooled andrecirculated to the pipe 144. Water or other coolant from the pipe 144is also passed through a pipe 147 to a pump 1% and sprayed a s a finemist by an atomizing nozzle 149 into the stream of compressible fluid inthe duct 65. The mist of Water or other coolant is carried through theduct 65, the plenum portion of the chamber 41, and the cooling chamber64 by the stream of compressible fluid. The strip work 32 can beimmersed in the atmosphere seal 42 without appreciable thermal shockwhen the mist of liquid coolant is employed.

The advantage of heating work in strip form in apparatus according tothe invention, which provides a sufliciently high rate of flow ofcompressible fluid adjacent the strip work that the ratio of heattransferred by convective heat transfer to heat transferred by radiativeheat transfer is at least 2:1, w'll be apparent from a briefconsideration of FIGS. 1719. Curves A, B and C of FIG. 17 show thetemperature of strip work, from an inlet temperature of T to a finaltemperature of T,, as a function of furnacing time, the total time beingS (represented by the dotted line). Curve A shows such time-temperaturerelationships when the compressible fluid is circulated through theheating chamber at a velocity just less than that required to causeflutter of the strip work, and the temperature of the compressible fluidis T Curve B is similar to curve A, but rep resents the lower limit ofoperation of apparatus according to the invention, where the velocity ofheated compressible fluid has been reduced to an extent such that theratio of heat transferred by convective heat transfer to heattransferred by radiative heat transfer is 2:1. When the velocity ofcompressible fluid flow has been decreased, as in the situationrepresented by curve B, it is necessary to raise the fluid temperatureto a temperature T higher than T in order to heat the strip work to thetemperature T in time S, because the overall coefficient of heattransfer has been lowered. When the compressible fluid velocity islowered still further below that necessary to effect heating asrepresented by curve B, the overall coeflicient of heat transfer to thestrip work is still further reduced, so that a still higher fluidtemperature T is required to heat the work from T to T, in true S. Thissituation is represented by curve C of FIG. 17, which is a typical curvefor rapid, high thermal head heating of strip work in apparatus knownprior to the instant invention.

By reference to FIG. 18, where the portions of curves A, B and Cadjacent their intersections with the dotted line T; are shown on anenlarged scale, it will be seen that the temperature differences to beexpected in strip Work as a result of variations of residence time inthe furnace are substantially higher in high thermal head furnacingwhere radiative heat transfer predom nates than in furnacing operationsconducted in apparatus according to the invention. Substantially smallertemperature differences result from residence time variations when theratio of convective to radiative heat transfer is at least 2: 1, asrepresented by curve B, and the differences are even less under thelimiting condition at a fluid velocity just short of that at whichresonant flutter occurs, as represented by curve A. Since it isimpossible to control residence time of the str'p in a furnaceabsolutely, apparatus according to the invention has the advantage overhigh thermal head furnacing apparatus that variations in residence timeaffect final work temperature less. For example, if the furnacingapparatus is designed to provide a residence time of S plus (E minus S)or, m'nus (S minus D), the improved temperature control (overconventional high thermal head furnacing) achieved by operating at acompressible fluid velocity sufficient to give a ratio of heattransferred by convective heat transfer to heat transferred by radiativeheat transfer of at least 2:1 is represented by the increment AT; or ATS milarly, the improved temperature control achieved by operatingapparatus according to the invention at a compressible fluid velocityjust short of that required to cause resonant flutter is represented bythe increment AT or AT In a specific instance it has been determinedthat an increase by 10 percent in residence t'me of brass strip in afurnace will cause an increase in temperature of the strip ofapproximately 50 F. in a high thermal head furnace, but of only about 25F. when, according to the invention, a compressible fluid velocity suchthat the ratio of heat transferred by convection to heat transferred byrad ation is approximately 3:1 is used. Variations in gauge of the stripwork being heat treated have the same general effect as variations inresidence time. A 10 percent decrease in gauge is approximately theequivalent of a 10 percent increase in residence time, and a 10 percentincrease in gauge is approximately equivalent to a 10 percent decreasein residence time. Thus, in FIG. 18, AT AT AT and AT; represent thevariations in temperature to be expected as a consequence of gaugevariations, if the furnace residence times remain constant.

Variations in strip emissivity cause even greater variations in stripdischarge temperature from a high thermal head furnacing apparatus thando variations in gauge of the strip or residence time. FIG. 19 shows onan enlarged scale the portions of curves A and C adjacent theirintersections with the dotted line representing T and, also, companioncurves representing temperatures if the strip emissivity increases from0.2 (curves A and C) to 0.3, or decreases to 0.1. As has been stated,such variations in emissivity must be anticipated. As shown in FIG. 19,when apparatus according to the invent'on is operated with acompressible fluid velocity just short of that which would causeresonant flutter, a substantially decreased temperature variation (AT indischarge strip temperature results from emissivity variations between0.1 and 0.3 than with convent'onal high thermal head furnacing (AT Therelationships for curve B of FIG. 17 are not represented in FIG. 19 inorder to avoid confusion; the band of temperatures to be expected fromvariations in emissivity from 0.1 to 0.3 would be narrower than thatshown with curve C, but somewhat broader than that shown with curve A.In a specific instance it has been determ'ned that emissivity variationsbetween 0.1 and 0.3 in strip Work will result in temperature variationsin a high thermal head furnacing apparatus of more than 350 F., while,in apparatus according to the invention, operated to give a ratio ofheat transferred by convection to heat transferred by radiation of about3.0, the temperature var ation will be less than F.

It will be observed from FIG. 19 that the temperature range resultingfrom emissivity variations decreases in magnitude with increases infurnacing time and strip temperature. This is true for the range withcurve A, as well as for the range with curves B and C. If furnacing werecontinued for an infinite time, the three curves would become identical,and the strip work temperature at all points would equal the furnacetemperature. Decreased temperature variations as a result of differencesin strip emissivity in apparatus according to the invention are achievednot only because radiation is a smaller factor in heating the stripwork, but also because heating is conducted so that a much closerapproach to temperature equilibrium in the strip work is achieved inapparatus according to the invention than in high thermal headfurnacing. It will be observed from FIG. 17 that the shapes of the threecurves A, B and C are generally the same, except that they approachdifferent limits, T T and T respectively. The proximity of striptemperature to equilibrium conditions after time S (or at discharge)depends upon temperature head (T minus Tf, T minus T or T minus Trespectively), the smaller the temperature head the closer the approachto temperature equilibrium in the strip. However, a high compressiblefluid velocity is required to heat the strip work in time S if the fluidis at a low temperature.

In one aspect, therefore, the invention contemplates a method for thecontinuous heating of strip work by heat transfer between a compressiblefluid and the strip while the compressible fluid is forced to flow, in adirection generally axial of the strip, at a velocity sufficiently highthat the ratio of heat transferred by convection to heat transferred byradiation is at least 2:1. It is preferred that the temperaturedifference, in degrees F, between the compressible fluid and strip workafter heating thereby, be not more than about 30 percent of the numberof degrees that the strip work is heated, and most preferred that suchtemperature difference be not more than about 25 percent. It is alsopreferred to use a compressible fluid velocity that is not onlysufficient to give a ratio, as indicated, but also sufficient toaccomplish the necessary heating in the available time at a relativelylow fluid temperature, as indicated. In this way, a close approach totemperature equilibrium in the strip at the time of discharge thereoffrom the heating apparatus is achieved.

The following example is presented solely for the purpose of furtherillustrating and disclosing the invention, and is in no way to beconstrued as a limitation thereon.

Example A furnace of the type shown in FIG. 2 with a duct heatingchamber having a cross-section area of 1.5 square inches was used toanneal samples of cartridge brass Work in strip form, 0.005 inch thick,and having different histories and different grain sizes. The brassstrip, as received, had a grain size, in millimeters, ranging in varioussamples from 0.010 to 0.090, and was reduced in various samples from 83percent to 40 percent by cold rolling prior to furnacing. Each sample ofstrip, after reduction, was subjected to furnacing for fifteen to thirtyseconds, with flue gas at temperatures from 1050" F. to 1450 F. passedthrough the heating chamber at velocities ranging from 120 to 150 linealfeet per second. The annealed work was then examined visually under amicroscope, and grain size estimated.

The results of these tests are plotted as a curve shown on FIG. 20 whichindicates generally the relationship between annealed grain size and thefinal temperature reached by the strip prior to emerging from theheating chamber. To one skilled in the art of annealing cartridge brassby conventional slow methods of heating, often a matter of hours, thisgrain size vs. strip temperature relationship has been found to be ofextreme interest and is believed to be new in the art in that higherstrip temperatures were used for the grain size produced than heretoforebelieved possible. It will be observed from FIG. 20 that average grainsize, in millimeters, can be expected to vary as a function of annealingtemperature from about 0.01 at 1100 F., to about 0.02 at 1200, 0.034 at1300, 0.056 at 1400 and 0.07 at 1450 F., when heating from cold to finaltemperature in 30 seconds or less total time.

It has been determined that emissivity of brass strip can be expected toaverage about 0.2, but to vary between about 0.1 and 0.3. It has beendemonstrated that it is not feasible to provide uniform grain size bycontinuous, rapid radiative heat transfer annealing. For example, ifbrass strip were pre-heated to 800 F. and then heated by radiative heattransfer to an average of 1200 F. annealing temperature, even at therelatively low rate of 12,000 pounds per hour of 0.025 thick by 26" widestrip in a heating chamber 24 feet long, portions thereof having anemissivity of 0.3 would be heated approximately to 1320 F, whileportions thereof having an emissivity of 0.1 would be heated only toabout 1040 F. Thus, the total temperature variation to be expected inthe strip work would be 280 F. It will be seen by reference to FIG. 20that at least a four-fold variation in grain size would be expected toresult from such temperature variations.

In apparatus according to the invention, convective heat transfer hasbeen made to predominate over radiative heat transfer to such an extentthat continuous rapid annealing of brass strip to uniform grain size ismade possible, notwithstanding variations in strip surface emissivity.

it has also been found that high thermal head furnacing cannot beadapted to the type of apparatus contemplated by this invention and givethe degree of control desired. For example, even when a high velocitycompressible fluid is used in a heating chamber for heating brass stripto about 1200 F. according to the invention, it is not known to bepossible to achieve sufliciently close temperature control of the workwhen the temperature head is more than about 300. The term temperaturehead is used herein, and in the appended claims, to refer to thetemperature difference, in degrees F., between work discharged from aheating or cooling chamber and the temperature of compressible fluidssupplied thereto. Preferred results in the form of more uniformdischarge temperature for the work are achieved when the temperaturehead is not greater than 250, and optimum results are achieved in themost desired instance when the temperature head is not greater than 200.Uniformity of final temperature of brass strip work may be estimated onthe basis of finished grain size. In general, the smaller thetemperature head, the longer the furnacing time required to heat thework to the desired temperature. Therefore, a temperature head of atleast 25 is usually preferred, and at least 50 is most desired.

It will be apparent that various changes and modification can be madefrom the specific details discussed and shown in the attached drawingswithout departing from the spirit of the invention. As a specificexample, compressible fluid velocities in apparatus according to theinvention have been discussed as having an upper limit just below thatvelocity which causes resonant flutter. Because flutter is believed tobe a resonant condition, however, it is to be expected that stripstability would be achieved throughout a range of fluid velocitieshigher than those at which resonant flutter occurs. In order to achievesuch higher velocities, it would be necessary to stabilize the strip,for example by means of removable heavy plates on either side thereof,as fluid velocities are increased through and beyond those which causeresonant flutter. The plates or other stabilizing means could then bedischarged, for example from a heating chamber, and operation conducted,as previously described, at fluid velocities within a range beyond thefirst resonant flutter point. It has been found experimentally to bepossible to achieve compressible fluid velocities in apparatus accordingto the invention sufficiently high that the ratio of heat transferred byconvective heat transfer to heat transferred by radiative heat transferis as high as 6:1 for brass, and 20:1 for aluminum before resonantflutter occurs. Even higher ratios could be achieved using fluidvelocities beyond the first resonant flutter point, or on materialshaving extremely low emissivity (mirror-like surfaces).

We claim:

1. A method for heat treating metal strip work which comprises passingthe work through an enclosed heating zone and then through a separateand enclosed cooling zone, causing a heated compressible fluid to flowthrough the heating zone generally longitudinally of the work, at avelocity sufiiciently high that the ratio of heat transferred to thework by convective heat transfer to heat transferred by radiative heattransfer is at least 2:1 and at a temperature which is higher than thetemperature at which the strip enters the heating zone by from 1.0 to1.3 times the number of degrees the strip is heated therein, and causinga cool compressible fluid to flow at high velocity through the coolingzone generally longitudinally of the work, and wherein the work isunsupported during its travel from entering the heating zone to leavingthe cooling zone, but is maintained under tension by forces appliedexteriorly of said zones.

2. A method for heat treating cold worked metal strip work to effectgrain recrystallization and consequent annealing, which method comprisespassing the Work in a substantially vertical direction through anenclosed heating zone and then through a separate and enclosed coolingzone, causing a heated compressible fluid to flow within the heatingzone in heat transfer relationship with the work to effect heatingthereof, in a heating region, from below a minimum temperature at whichthe work is subject to marking due to roll pick-up and to a temperaturesufliciently high to cause grain recrystallization and consequentannealing, and cooling the work, in a cooling region of the coolingzone, to below the minimum temperature at which the work is subject tomarking by roll pick-up, by causing a cool compressible fluid to flow athigh velocity within the cooling zone in heat transfer relationship withthe work and by maintaining a body of liquid water in contact with thework adjacent the strip discharge end of the cooling region, and whereinthe work is unsupported during its travel from entering the heatingregion to leaving the cooling region, but is maintained under tension byforces applied exteriorly of said regions. I 3. Arnethod forheat-treating cold worked metal strip work to effect grainrecrystallization and consequent annealing, which method comprisespassing the work in a substantially vertical direction through anenclosed heating zone, and then through a separate and enclosed coolingzone, causing a heated compressible fluid to flow within the heatingzone in heat transfer relationship with the work to effect heatingthereof, in a heating region, from below a minimum temperature at whichthe work is subject to marking due to roll pick-up and to a temperaturesufficiently high to cause grain recrystallization and consequentannealing, and causing a cool compressible fluid to flow at highvelocity Within the cooling zone in heat transfer relationship with thework and effecting cooling thereof, in a cooling region, to below theminimum temperature at which the work is subject to marking by rollpick-up, and wherein the work is unsupported during its travel fromentering the heating region to leaving the cooling region, but ismaintained under tension by forces applied exteriorily of said regions.

4. A method for heat treating cold worked metal strip work to effectgrain recrystallization and consequent annealing, which method comprisesheating the work from a temperature of T to T, by passing it in asubstantially vertical direction through an enclosed heating zone andthen through a separate and enclosed cooling zone, causing a heatedcompressible fluid at a temperature not more than 1.3 times (T -T higherthan T to flow at high velocity through the heating zone generallylongitudinally of, and in heat transfer relationship with, the work toeffect heating thereof, in a heating region, from below a minimumtemperature at which the work is subject to marking due to roll pick-upand to a temperature sufliciently high to cause grain recrystallizationand consequent annealing, and causing a cool compressible fluid to flowat high velocity through the cooling zone general- 1y longitudinally of,and in heat transfer relationship with, the work and effecting coolingthereof, in a cooling region, to below the minimum temperature at whichthe work is subject to marking by roll pick-up, and wherein the work isunsupported during its travel from entering the heating region toleaving the cooling region, but is maintained under tension by forcesapplied exteriorly of said regions, the velocity of the heatedcompressible fluid being sufficiently high that the ratio of heattransferred to the work by convective heat transfer to heat transferredby radiative heat transfer is from about 2:1 to about 20:1.

5. A method for heat treating cold worked metal strip work to effectgrain recrystallization and consequent annealing, which method comprisespassing the work in a substantially vertical direction through anenclosed heating zone and then through a separate and enclosed coolingzone, causing a heated compressible fluid to flow through the heatingzone in heat transfer relationship with the work to effect heatingthereof, in a heating region, from below a minimum temperature at whichthe work is subject to marking due to roll pick-up and to a temperaturesufficiently high to cause grain recrystallization and consequentannealing, and causing a cool compressible fluid to flow at highvelocity through the cooling zone generally longitudinally of, and inheat transfer relationship with, the work and effecting cooling thereof,in a cooling region, to below the minimum temperature at which the workis subject to marking by roll pick-up, by directing a primarycompressible fluid stream into a fluid inlet end of, through, and from afluid outlet end of the cooling zone on each side of the strip work',and directing a secondary compressible fluid stream into each of theprimary streams at the fluid inlet end of the zone so that a mixture ofthe primary and secondary streams flows through, and from the zone athigh velocity, and wherein the work is unsupported during its travelfrom entering the heating region to leaving the cooling region, but ismaintained under tension by forces applied exteriorly of said regions. gV

6. A method according to claim S Wherein the metal strip is maintainedat a tension between 50 percent and 100 percent of that which will causea strip elongation of of 1 percent under the conditions prevailing.

7. A method according to claim 5 wherein the volume ratio of fluidpassing in said primary streams to fluid passing in said secondarystreams is from 1.5:1 to 3:1.

8. A method for heat treating cold worked metal strip work to effectgrain recrystallization and consequent annealing, which comprisespassing the work through an enclosed heating zone and then through aseparate and enclosed cooling zone, controlling the temperature at whichthe work enters the heating zone to one below the minimum at which thework is subject to marking due to roll pick-up, causing a heatedcompressible fluid to flow within the heating zone in heat transferrelationship with the work and to heat the work to a temperaturesufficiently high to cause grain recrystallization and consequentannealing, controlling the fluid velocity within the heating zone to onesufliciently high that the ratio of heat transferred to the work byconvective heat transfer to heat transferred by radiative heat transferis at least 2:1, controlling the fluid temperature within the heatingzone to one higher than the temperature at which the strip enters theheating zone by from 1.0 to 1.3 times the number of degrees the strip isheated therein, and causing a cool compressible fluid to flow at highvelocity within the cooling zone and cooling the work to a temperaturebelow the minimum at which the work is subject to marking by rollpick-up, and wherein the work is unsupported during its travel fromentering the heating zone to leaving the cooling zone, but is maintainedunder tension by forces applied exteriorly of said zones.

9. A method as claimed in claim 8 wherein the work is pre-heated to asub-critical temperature before it enters the heating zone.

.10. ;A method as claimed in claim '8 wherein the work ,is furthercooled after leaving the cooling zone.

11. A method as claimed in claim 8 wherein a body .of liquid water ismaintained withinthe cooling zone ,in contact with the work adjacent thestrip discharge end of the zone.

12. Apparatus for continuous rapid heat treating of metal work in stripform comprising a vertically extend- .ing heating chamber havingcontinuous, closed sidewalls and open ends for the passage of worktherethrongh, an adjacent, aligned, vertically extending cooling chamberhaving continuous closed sidewalls and open ends, for thepassage of worktherethrough one of the open ends of said cooling chamber beingpositioned in spaced, aligned relationship relative to one of the openends of Said heating chamber, vertically aligned cooperating rollsexterior of said heating and cooling chambers for conveying strip workthrough said heating chamber and then through said cooling chamber alonga generally vertical work path, the work being unsupported at all pointswithin said chambers, inlet means for introducing a heated compressiblefluid into said heating chamber for circulation in heat transferrelationship with work therein, means for withdrawing compressible fluid.from said heating chamber, means for circulating withdrawn compressiblefluid and for returning it tosaid inlet means, means ,for heating thecirculating compressible fluid, inlet means for introducing a coolcompressible fluid into said cooling chamber for circulation in heattransfer relationship with work therein, and means for withdrawingcompressible fluid from said cooling chamber, and effective to preventthe flow of compressible fluid from said cooling chamber into saidheatingchamber.

13. Apparatus for continuous rapid heat treating of metal work in stripform comprising avertically extend- 35 ing heating chamber havingcontinuous, closed side :walls and upper and lower open ends for thepassage of work therethrough, an adjacent, aligned, vertically extendingcooling chamber having continuous closed side Walls and upper and loweropen ends for the passage of workthere- 357,575

through, the upper openend of sa Cooling Chamber being positioned inspaced, aligned relationship relative to the lower open end of saidheating chamber, a water chamber vertically aligned relative to thelower open end of said cooling chamber, vertically aligned cooperatingrolls exterior of said heating chamber and one being contained withinsaid water chamber for conveying strip work through said heating chamberand then through said cooling chamber along a generally verticalworkpath, the work being unsupported at all points within said heatingand cooling chambers, inlet means ,for introducing a heated compressiblefluid into .said heating chamber for circulation in heat transferrelationship with work therein, means for withdrawing compressible fluidfrom said heating chambenmeans for circulating withdrawn compressiblefluid and for returning it to said inlet means, means for heating thecirculating compressible fluid, inlet means ,for introducing a coolcompressible fluid into said cooling chamber for circulation in heattransfer relationship with work therein, and means for withdrawingcompressible fluid from said cooling chamber and effective to preventthe flow o-fcompressible fluid from said cooling chamber into saidheating chamber.

References Cited in the file of this patent UNITED STATES PATENTS2,079,867 Meyers May 11, 1937 2,144,919 Gautreau Jan. 24, 1939 2,232,391Kellar Feb. 18, 1941 2,424,034 Hopper July 15, 1947 2,428,362 Egge Oct.7, 1947 2,458,525 Nachtman Jan. 11, 1949 2,534,973 Ipsen et a1. Dec. 19,1950 2,546,538 Erhardt Mar. 27, 1951 2,686,639 Campbell Aug. '17, 19542,779,584 Edvar Jan. 29, 31957 FOREIGN PATENTS Great Britain Sept. 21,193 1

2. A METHOD FOR HEAT TREATING COLD WORKED METAL STRIP WORK TO EFFECTGRAIN RECYSTALLIZATION AND CONSEQUENT ANNEALING, WHICH METHOD COMPRISESPASSING THE WORK IN A SUBSTANTIALLY VERTICAL DIRECTION THROUGH ANENCLOSED HEATING ZONE AND THEN THROUGH A SEPARATE AND ENCLOSED COOLINGZONE, CAUSING A HEATED COMPRESSIVEL FLUID TO FLOW WITHIN THE HEATINGZONE IN HEAT TRANSFER RELATIONSHIP WITH THE WORK TO EFFECT HEATINGTHEREOF, IN A HEATING REGION, FROM BELOW A MINIMUM TEMPERATURE AT WHICHTHE WORK IS SUBJECTED TO MARKING DUE TO ROLL PICK-UP AND TO ATEMPERATURE SUFFICIENTLY HIGH TO CAUSE GRAIN RECRYSTALLIZATION ANDCONSEQUENT ANNEALING, AND COOLING THE WORK, IN A COOLING REGION OF THECOOLING ZONE, TO BELOW THE MINIMUM TEMPERATURE AT WHICH THE WORK ISSUBJECT TO MARKING BY ROLL PICK-UP, BY CAUSING A COOL COMPRESSIBLE FLUIDTO FLOW AT HIGH VELOCITY WITHIN THE COOLING ZONE IN HEAT TRANSFERRELATIONSHIP WITH THE WORK AND BY MAINTAINING A BODY OF LIQUID WATER INCONTACT WITH THE WORK ADJACENT THE STRIP DISCHARGE END OF THE COOLINGREGION, AND WHEREIN THE WORK IS UNSUPPORTED DURING ITS TRAVEL FROMENTERING THE HEATING REGION TO LEAVING THE COOLING REGION, BUT ISMAINTAINED UNDER TENSION BY FORCES APPLIED EXTERIORLY OF SAID REGIONS.